Albright hereditary osteodystrophy (AHO) is an autosomal dominant disorder characterized by short stature, obesity, subcutaneous ossifications and brachydactyly. Some family members have these features in association with resistance to multiple hormones which activate Gs-coupled receptors (pseudohypoparathyroidism type Ia, PHP Ia) while others present with the somatic features alone (pseudopseudohypoparathyroidism, PPHP). Peripheral tissues from most affected patients have a 50% deficiency in Gs-alpha subunit function and/or expression in peripheral tissues (both PHP Ia and PPHP). We have identified a number of heterozygous inactivating mutations of the Gs-alpha gene in affected patients (both PHP Ia and PPHP). Examination of the biochemical properties of these mutants and others demonstrates that arginine 258 in the switch 3 region interacts with a residue in the helical domain and that this interaction is critical for guanine nucleotide binding. This residue and glutamate 259 are also important for G protein activation, although by different mechanisms. It has been proposed that tissue-specific imprinting of the Gs-alpha gene may explain the observation that maternal transmission of the Gs defect leads to offspring with PHP Ia while paternal transmission leads to PPHP. We demonstrated in Gs-alpha 'knockout' (GsKO) mice that the gene is imprinted in a tissue- specific manner such that in certain tissues the expression of the paternal allele is less than the paternal allele, and this likely explains the variable and tissue-specific hormone resistance in these mice (and likely in AHO). The following studies are ongoing: 1. characterization of allele-specific methylation around the Gs-alpha gene and delineating the molecular mechanism of tissue-specific imprinting of this gene 2. Characterization of the metabolic defects (increased insulin sensitivity, abnormal fat metabolism) in Gs-alpha mice and their underlying mechanisms 3. Studies to determine the genetic defects in other forms of parathyroid hormone resistance.